Delayed coking is a process used in the petroleum refining industry for increasing the yield of liquid product from heavy residual oils such as vacuum resid.
In delayed coking, the heavy oil feed is heated in a furnace to a temperature at which thermal cracking is initiated but is low enough to reduce the extent of cracking in the furnace itself. The heated feed is then led into a large drum in which the cracking proceeds over an extended period of residence in the drum. The cracking produces hydrocarbons of lower molecular weight than the feed which, at the temperatures prevailing in the drum, are in vapor form and which rise to the top of the drum where they are led off to the downstream product recovery unit with its fractionation facilities. The thermal cracking of the feed that takes place in the drum also produces coke, which gradually accumulates in the drum during the delayed coking cycle. When the coke reaches a certain level in the drum, the introduction of the feed is terminated and the cracked products remaining in the drum are removed by purging with steam. After this, the coke is quenched with water, the drum is depressurized, the top and bottom heads are opened, and then the coke is discharged through the bottom head of the drum through use of a high pressure cutting water system. The cracking cycle is then ready to be repeated. Typically the process itself is achieved by heating the heavy oil feed to a temperature in the range that permits a pumpable condition in which it is fed into the furnace and heated to a temperature in the range of 380 to 525° C.; the outlet temperature of a coker furnace is typically around 500° C. with a pressure of 4 bar. The hot oil is then fed into the coke drum where the pressure is held at a low value in order to favor release of the vaporous cracking products, typically ranging from 1 to 6 bar, more usually around 2 to 3 bar. Large volumes of water are used in the quench portion of the coking cycle: one industry estimate is that for a typically large coke drum about 8 m in diameter and 25 m high, about 750 tonnes of water are required for quenching alone with even more required for the cutting operation after the drum is opened and the coke discharged. A useful and widely cited summary of the delayed coking process is available online in “Tutorial: Delayed Coking Fundamentals”, Ellis et al, Great Lakes Carbon Corporation, Port Arthur, Tex., AlChE 1998 Spring National Meeting, New Orleans, La., 8-12 Mar. 1998, Paper 29a, Copyright ©1998 Great Lakes Carbon Corporation.
Delayed coking coke drums are conventionally large vessels, typically at least 4 and possibly as much as 10 m in diameter with heights of 10 to 30 m or even more. The drums are usually operated in twos or threes with each drum sequentially going through a charge-quench-discharge cycle, with the heated feed being switched to the drum in the feed phase of the cycle. The drums are typically made of unlined or clad steel, with base thicknesses that can range from about 10 to 30 mm thick. The internal cladding thickness is nominally 1-3 mm and is used for protection against sulfur corrosion. The present common commercial practice is to use 401S clad or unclad CS, C-1/2 Mo, or low chromium drums for delayed coking service. In form, the drums comprise vertical cylinders with either an ellipsoidal or hemispherical top head and a conical bottom head. The bottom head has either a flange or, alternatively, a mechanical valve arrangement as described, for example, in U.S. Pat. No. 6,843,889 (Lah). The feed inlet and steam/water connections are located in this lower conical section of the vessel. Operating envelopes and inspection/repair strategies are the mechanisms used to manage fatigue cracking in this equipment.
Delayed Coker coke drums are inherently exposed to pressure boundary fatigue cracking due to the thermal stresses imposed on the steel primarily during the quench/fill process. The drums are prone to thermal fatigue due to the through-wall thermal stresses that are developed prior to the drum reaching steady state. Additionally, at the skirt-to-shell junction, the transient temperature differentials between the pressure boundary and the skirt also set up high stresses that can lead to weld and base metal cracking. This is a transient effect, and data analysis has shown that the other delayed coking steps (e.g., drum warm-up, feed introduction, coking, steam out, etc.) have less impact on pressure boundary stresses. As noted by Ellis, op. cit., the rate of cooling water injection is critical. Increasing the flow of water too rapidly can “case harden” the main channels up through the coker without cooling all of the coke radially across the coke bed. The coke has low porosity which then allows the water to flow away from the main channels in the coke drum, leading to the problem of drum bulging during cool down. If the rate of water is too high, the high pressure causes the water to flow up the outside of the coke bed cooling the wall of the coke drum. Coke has a higher coefficient of thermal expansion than does steel (154 for needle coke versus 120 for steel, cm/cm/° C.×10−7). While drum support systems such as that described in U.S. Pat. No. 8,221,591 (de Para) may be capable of reducing the mechanical stresses generated by the differential cooling, it would nevertheless by desirable to minimize the transient thermal stress in both the coke drum Shell/cone as well as at the skirt-to-shell junction.